1. Field of the Invention
Some natural subsurface waters (fresh or salty) at high pressures (geopressured or hydrostatic) contain dissolved methane and other constituents of natural gas in commerically useful concentrations. These natural gas constituents may be dissolved in the waters, held as gas bubbles which have been entrapped in porous sandstone or shale, or occur as solid hydrocarbon hydrates. For simplicity in our following descriptions, the term "brine" defines subsurface water solutions, "methane" defines hydrocarbon-gas mixtures which are being delivered from the brines, and "sandstone" defines porous solid formations which hold the brine. This invention encompasses a method and the devices for releasing methane from the natural gas constituents listed above through the use of methane ebullition brought about by pressure reduction, plus methane sweeping in which bubbles of steam or other gases carry off methane as they move up through the brine. A critical aspect of this invention is that the processes are carried out in situ (subsurface) so that the environmentally troublesome brine is never brought to the surface in order to accomplish the methane recovery. Likewise, steam can be recovered in situ in certain cases for which surface processing of brine would normally be required e.g., with geopressured brines. The pressure is lowered by pumps as brine is withdrawn from the sandstone, processed for methane and steam recovery, and reinjected into sandstone elsewhere; alternatively the pressure beneath a dome (natural, or artificially created as in this patent) in a region of geopressured can be reduced so that the whole originally geopressured brine reservoir can deliver its methane and steam to surface facilities. New subsurface engines and pumps which utilize energy both from natural in situ forces and from surface engines are described. These devices allow energy-efficient recovery of the methane and steam, and offer the possibility of electric power generation subsurface for down-hole devices.
2. Prior Art
A. "Natural Gas Resources of the Geopressured Zones in the Northern Gulf of Mexico Basin," pp. 17-33, by P. H. Jones in "Natural Gas from Unconventional Sources," Board of Mineral Resources, Commission on Natural Resources, National Academy of Sciences, Washington, DC, 1976. Jones recognizes the variation of methane solubility with water pressure, the importance of deep hydrocarbons as a source of methane, and the possibility of generating artificially filled gas reservoirs at natural domes (p. 29). Jones proposes that suitable conditions for such artificial filling would be created if pressures in the sandstone were dropped some 50% by drawing brine to the surface (p. 6), and suggests a way to search for the artificial gas caps after they had been created (p. 29). However, Jones does not identify means to develop artificial gas caps other than by bringing large amounts of brine to the surface, and he does not propose the creation of artificial domes for gas collection.
B. "Method for Increasing the Recovery of Natural Gas from a Geo-Pressured Aquifer," Cook, Jr., et al., U.S. Pat. Nos. 4,040,487 and 4,042,034, Aug. 9 and 16, 1977. These inventors recognize the value of reducing brine pressures in sandstone reservoirs to release methane within the reservoirs, but the method they propose requires bringing large amounts of brine to the surface, and the method recovers 14% or less (their calculations) of the methane present initially in the brine they process. By contrast the process of this invention could produce up to 80% of the methane present.